Which of the following is a substantive test that an auditor most likely would perform to verify that accounts payable are not understated? a. Investigating the open purchase order file to ascertain that prenumbered purchase orders are used and accounted for. b. Confirming accounts payable balances with known suppliers who have zero balances. c. Vouching selected entries in the accounts payable subsidiary ledger to purchase orders and receiving reports. d. Receiving the client's mail, unopened, for a reasonable period of time after the year-end to search for unrecorded vendor invoices. e. Reconciling vendor statements to receiving reports to identify items received after the balance sheet date